The invention relates to a converter comprising circuit elements for chopping a DC voltage, in which switch-on phases of the circuit elements are alternating, and comprising a circuit assembly with resonant circuit elements which is used for processing the chopped DC voltage and for producing an output voltage.
Such load resonant converters preferably represent switching power supplies which are used for supplying DC voltage to a load connected to the output of the switching power supply. In such switching power supplies, an AC voltage present on the input is first rectified to obtain a converter input DC voltage. However, the invention also relates to converters to whose inputs a DC voltage is supplied directly from a DC voltage source. Such a converter can also be used for operation of gas discharge lamps. The converter input DC voltage is chopped by means of a bridge circuit comprising circuit elements. The chopped DC voltage is applied to a circuit assembly with resonant circuit elements i.e. with inductive and capacitive reactance elements, so that a substantially sinusoidal AC current flows in the circuit assembly. At least one inductive and at least one capacitive resonant circuit element are to be available. A load may be connected to the output of the circuit assembly and thus to the output of the converter. By adapting the switching frequency, load changes and input voltage variations are adapted to. Converters with resonant circuit elements i.e. resonant converters, enable the circuit elements to operate with high switching frequencies and thus relatively small-volume and light devices compared to the possible power output can be realized. When resonant converters are used, also a so-called zero-voltage switching operation (ZVS) is enabled with little circuit expenditure. ZVS operation is here understood to mean that circuit elements are switched on (brought to the conducting state) with a lowest possible voltage of the circuit element, preferably close to zero volts. In the ZVS mode the circuit assembly with the resonant circuit elements has an inductive input impedance considered from the side of the circuit elements. In the case of a ZVS mode, MOSFET transistors are customarily used as circuit elements. With converters realized in this way the operation with a capacitive load is to be avoided. Such a converter mode leads to increased switching losses and may even cause the destruction of converter circuit elements. Therefore, means are known to be provided for determining the type of converter load (inductive or capacitive) with such load resonant converters.
From EP 0 430 358 A1 is known a converter circuit arrangement for gas discharge lamps in which the type of converter load is determined in the way described above. The circuit arrangement includes a half bridge with circuit elements for chopping a DC voltage. On the output side of the half bridge is arranged a circuit assembly including resonant circuit elements, which assembly is used for supplying a voltage to a discharge lamp. Here too, operation with a capacitive converter load is to be avoided. For this reason the phase difference between the voltage applied to the circuit assembly and the current flowing into the circuit assembly is indirectly monitored by monitoring the current flowing into the circuit assembly.
It is an object of the invention for the converter defined in the opening paragraph to propose a further type of converter load monitoring, which type can be changed with the least possible circuit expenditure and the least possible measuring losses.
The object is achieved in that in a dead time phase before a circuit element is switched on, the voltage present on the circuit element is compared with a threshold and from the comparison result there is ascertained whether an inductive or capacitive converter load is present.
Expensive measurements of phase differences are avoided in this manner. Furthermore, only voltage measurements are necessary and no current measurements linked with losses. If necessary, in case of an undesired type of converter load the normal converter operation may, for example, be broken off and a new start sequence may be made. Determining the type of converter load may be effected very rapidly in this manner, so that undesired converter operating modes can be counteracted with counter measures very rapidly. Determining the type of converter load in accordance with the invention is also suitable for high switching frequencies.
In an embodiment of the invention the comparison with the threshold takes place in each dead time phase before either of the circuit elements is switched on. The time space up to the detection of an undesired converter operating mode is kept smallest possible in this manner.
The object is achieved in that, during a dead time phase, the derived value of the voltage present on a circuit element is determined and with the aid of the determined derived value there is ascertained whether an inductive or a capacitive converter load is present.
Alternatively, it is possible to include a time-average value for the derived value of the voltage present on a circuit element and use this time-average value for the comparison. Expensive measurements of phase differences are avoided in this manner. Furthermore, only voltage measurements are necessary and no current measurements linked with losses. If necessary, in case of an undesired type of converter load the normal converter operation may, for example, be broken off and a new start sequence may be made. Determining the type of converter load may be effected very rapidly in this manner, so that undesired converter operating modes can be counteracted with counter measures very rapidly. Determining the type of converter load in accordance with the invention is also suitable for high switching frequencies.
In an embodiment of the invention the evaluation of the derived value of the voltage present on a circuit element is made for each dead time phase and the comparison with the threshold is made before either of the circuit elements is switched on, i.e. the type of converter load is monitored cycle by cycle. The time space up to the detection of an undesired converter operation mode is kept shortest possible in this manner.
The invention also relates to an accordingly arranged control unit, more particularly an integrated circuit for controlling at least one of the converter circuit elements.